Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink, whereas other areas will not accept ink.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy ink in the photo-exposed areas (negative working material) or in the non-exposed areas (positive working material) on an ink-repelling background.
Nowadays methods are known for making printing plates involving the use of imaging elements that are thermo-sensitive rather than photo-sensitive. A particular disadvantage of photo-sensitive imaging elements (such as described above) for making a printing plate is that they have to be shielded from daylight and need special handling conditions, e.g. so-called darkroom conditions. Furthermore, it is claimed that thermal plates have a higher resolution and higher tone range capabilities. The trend towards thermo-sensitive printing plate precursors is clear when observing present evolutions of the market (cfr. "Jetzt kommt die Thermoplatte in die Praxis: Startschu.beta. fur Computer-to-plate ist gefallen", in Deutscher Drucker, 32. Jahr, Nr. 24, Jun. 27, 1996, pp. g14-g15).
For example, EP-A 95.202.871.0, 95.202.872.8, 95.202.873.6 and 95.202.874.4 (all in the name of Agfa-Gevaert N. V.) disclose a method for making a lithographic printing plate comprising the steps of
(1) image-wise exposing to light a thermo-sensitive imaging element comprising (i) on a hydrophillic surface of a lithographic base an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophillic binder and (ii) a compound capable of converting light to heat, said compound being comprised in said image forming layer or a layer adjacent thereto; (2) and developing a thus obtained image-wise exposed element by rinsing it with plain water.
However, it is known that during the exposure of such imaging elements partial or total ablation may occur in at least the surface layer of these imaging elements. This may result in a deterioration of the lithographic properties of a so obtained lithographic plate, e.g. a decreased ink accepting behaviour of said ablated areas.
These ablation phenomena especially occur at a higher recording (or scanning) speed, or at a lower "recording time per microdot", further abbreviated as t.sub.p. Because of economical reasons, as a higher throughput means a higher productivity, there is a strong interest in the industry to increase the recording speeds, especially when using so-called "internal drum" or "flatbed" recording architectures. Now, for a given thermo-sensitive printing plate precursor, like those indicated in the previous paragraph, unwanted ablation or other unwanted side-reactions will occur when a given recording speed limit is surpassed. Due to the fact that a higher speed inherently requires a higher recording power (because only a smaller recording time per microdot is available for putting in a same energy) and due to the fact that a said printing plate precursor also shows a lower sensitivity at a lower recording time per microdot (called "sensitivity ir-reciprocity"), the required power when increasing the recording speed will become too high at a given speed for the imaging element to accommodate properly (wanted reaction) the heat/temperature build-up that goes with it. Then, a local burning or ablation or possibly another unwanted side-reaction takes place, which may cause a lowered "lithographic performance" (e.g. unwanted ink-uptake).
Now, a solution from the side of the imaging element has been proposed in EP-A-96.201.754.7 (in the name of Agfa-Gevaert N. V.).
This solution consists of using a specially designed top-coat layer which would reduce or prevent ablation or other unwanted side-reactions.
However, it would be more appropriate to have a hardware solution for this problem.